Double-layer capacitors, also referred to as ultracapacitors and super-capacitors, are energy storage devices that are able to store more energy per unit weight and unit volume than capacitors made with traditional technology.
Double-layer capacitors store electrostatic energy in a polarized electrode/electrolyte interface layer. Double-layer capacitors include two electrodes, which are separated from contact by a porous separator. The separator prevents an electronic (as opposed to an ionic) current from shorting the two electrodes. Both the electrodes and the porous separator are immersed in an electrolyte, which allows flow of the ionic current between the electrodes and through the separator. At the electrode/electrolyte interface, a first layer of solvent dipole and a second layer of charged species is formed (hence, the name “double-layer” capacitor).
Although, double-layer capacitors can theoretically be operated at voltages as high as 4.0 volts and possibly higher, current double-layer capacitor manufacturing technologies limit nominal operating voltages of double-layer capacitors to about 2.5 to 2.7 volts. Higher operating voltages are possible, but at such voltages undesirable destructive breakdown begins to occur, which in part may be due to interactions with impurities and residues that can be introduced during manufacture. For example, undesirable destructive breakdown of double-layer capacitors is seen to appear at voltages between about 2.7 to 3.0 volts. Double-layer capacitor can also provide high capacitance on the order of 1 to 5000 Farads in relatively small form factor housings.
Known capacitor electrode fabrication techniques utilize coating and extrusion processes. Both processes utilize binders such as polymers or resins to provide cohesion between the surface areas of the particles used.
In the coating process, the binder is dissolved in an appropriate solvent, typically organic, aqueous or blends of aqueous and organics, and mixed with the conductive material, such as carbon, to form a slurry. The slurry is then coated through a doctor blade or a slot die onto the current collector, and the resulting electrode is dried to remove the solvent. Among the numerous polymers and copolymers that can be used as a binder in the coating process, most of them suffer from a lack of stability when a subsequent electrolyte solvent is used to impregnate a final capacitor product. This is especially true when the solvent is an organic one and the working or storage temperature is higher than 65° C. Instability of the binder can lead to a premature failure of an electrode and, thus a capacitor.
Typical extrusion processes use the fibrillation properties of certain polymers to provide a matrix for embedded conductive material. Some of the polymers in the family of flouropolymers, such as polytetrafluoroethylene (PTFE), are particularly inert and stable in the common electrolyte solvents used in double-layer capacitors, even those using organic solvent at high working or storage temperatures. Thus, the stability of an electrode made using PTFE can be higher than those made with other binders. Polymers and similar ultra-high molecular weight substances capable of fibrillization are also commonly referred to as “fibrillizable binders” or “fibril-forming binders.” Fibril-forming binders find use with powder like materials. In one prior art process, fibrillizable binder and powder materials are mixed with solvent, lubricant, or the like, and the resulting wet mixture is subjected to high-shear forces to sufficiently fibrillize the binder particles. Fibrillization of the binder particles produces fibrils that eventually form a matrix or lattice for supporting the resulting composition of matter. The resulting dough-like material is calendared many times to produce a conductive film of desired thickness and density. In the prior art, the high shear forces can be provided by subjecting the mixture to an extruder.
In the prior art, the resulting additive based extruded product is subsequently processed in a high-pressure compactor, dried to remove the additive, shaped into a needed form, and otherwise processed to obtain an end-product for the needed application. For purposes of handling, processing, and durability, desirable properties of the end product typically depend on the consistency and homogeneity of the composition of matter from which the product is made, with good consistency and homogeneity being important requirements. Such desirable properties depend on the degree of fibrillization of the polymer. Tensile strength commonly depends on both the degree of fibrillization of the fibrillizable binder, and the consistency of the fibril lattice formed by the binder within the material. When used as an electrode, internal resistance of an electrode film is also important.
Internal resistance may depend on bulk resistivity—volume resistivity on large scale—of the material from which an electrode film is fabricated. Bulk resistivity of the material is a function of the material's homogeneity; the better the dispersal of the conductive carbon particles or other conductive filler within the material, the lower the resistivity of the material. When electrode films are used in capacitors, such as double-layer capacitors, capacitance per unit volume is yet another important characteristic. In double layer capacitors, capacitance increases with the specific surface area of the electrode film used to make a capacitor electrode. Specific surface area is defined as the ratio of (1) the surface area of electrode film exposed to an electrolytic solution when the electrode material is immersed in the solution, and (2) the volume of the electrode film. An electrode film's specific surface area and capacitance per unit volume are believed to improve with improvement in consistency and homogeneity.
Because fluoropolymers do not dissolve in most solvents, they are not suited for use as a binder in conventional solvent based coating processes. Because extrusion processes require large manufacturing equipment investments, it is often financially prohibitive for electrode manufacturers to adopt manufacturing processes that take advantage of the benefits of using fluoropolymers as a binder. As such, it would be desirable to use fluoropolymers in the manufacture of coated electrodes.